1. Field of the Invention
The present invention relates generally to a control apparatus for controlling the number of revolutions of an engine mounted on a vehicle. More particularly, the present invention relates to a control apparatus which enables a fuel-cut operation when the engine is running at a high speed, so as to prevent the engine from revolving excessively (i.e., overrunning).
2. Description of the Related Art
In general, a compulsive halt operation of fuel injection through injectors (i.e., fuel cut-off operation) is carried out to prevent the engine from revolving excessively, when the vehicular engine is running at a high speed. An engine speed sensor disposed in the engine continuously monitors the engine speed (NE) thereof, and transmits a detection signal to an engine speed controller. The controller includes memories which store a first determining value (Da) for use in determining timing of the halt operation of the fuel injection and a second determining value (Db), which is smaller than the first determining value (Da), for use in determining timing to resume the fuel injection. The controller comparers the first and second determining values with the engine speed (NE) detected by means of the engine speed sensor.
When the engine speed (NE) exceeds the first determining value (Da) for fuel injection halt, the controller carries out the fuel cut-off operation. When the engine speed (NE) drops below the second determining value (Db) for fuel injection resume, the controller resumes the fuel supplying operation. There is a width (i.e., hysteresis) between the first determining value (Da) and the second determining value (Db). The controller repeatedly carries out the operation to drop the engine speed due to the fuel cut-off and the operation to rise the engine speed due to the resumption of fuel supply. Consequently, the engine speed (NE) can be maintained in a range between the first and second determining values (Da and Db) to prevent the engine from overrunning, through the repeated execution of the above-mentioned operations. However, this traditional technique causes the engine speed (NE) to fluctuate largely between the first and second determining values (Da and Db), when the engine is running at a high speed. This large fluctuation of the engine speed lets a driver feel discomfort for driving a vehicle.
Japanese Unexamined Utility Model Publication No. 1-118142 discloses the technology which can minimize the fluctuation of the engine speed. According to the publication, the first and second determining values (Da and Db) are gradually decreased with the hysteresis therebetween being kept constant, while a predetermined period of time has elapsed since the engine speed reached a high speed level. As a result, the engine speed (NE) gradually decreases while it repeatedly fluctuates within the range between the determining values Da and Db.
However, everyone of the above-described conventional technologies generate small delay period of time .DELTA.t (i.e., time lag) which corresponds to the duration of the controller from reading the engine speed thereof to transmitting halt/resume instructional signal for fuel injection. This time lag is originated in the operational time of the controller or the cycle time of interrupt operation. Therefore, a small time lag is generated until the fuel cut-off operation is actually carried out since the engine speed exceeded the first determining value (Da) and until the fuel injection is actually resumed since the engine speed dropped below the second determining value (Db). The engine speed either continuously increases during the period of the time lag and overshoots the first determining value (Da), or continuously drops during the period of the time lag and then reaches below the second determining value (Db).
An ordinary vehicle includes a plurality of torsional dampers which are disposed between a drive shaft of the engine and a propeller shaft connected by drive wheels. The torsional dampers prevent the engine power caused by an acceleration or deceleration of the vehicle from being directly transmitted via the propeller shaft to the drive wheels. In other words, the dampers relieve the sudden fluctuation of the engine power. Further, the torsional dampers allow the drive shaft to displace or shift with respect to the propeller shaft, along an accelerating or decelerating direction of the driving wheels, due to self-swerve. Therefore, the dampers efficiently relieve the impact originated in the acceleration or deceleration.
The torsional dampers accumulate deformation energy or repulsion force generated by the swerve thereof when the speed of the driving wheels are accelerated or decelerated. Therefore, when the operation is reversed between the acceleration and deceleration operations, the torsional dampers not only dissolve the swerve but also assist the revolution of the engine drive shaft by the action of the accumulated deformation energy. As a result, the torsional dampers promote the engine speed to overshoot.
The fluctuation phenomena of the engine speed caused by the torsional dampers will now be described referring to FIGS. 11 and 12. FIG. 11 is a schematic view showing the relative position between an engine drive shaft 52 (i.e., a crank shaft) and a propeller shaft 51 connected to the drive wheels. FIG. 12 is diagram showing the correlation between the time and the engine speed (NE), condition of fuel cut-off and displacement of the engine drive shaft 52.
The propeller shaft 51 and drive shaft 52 rotate in the clockwise direction in FIG. 11. When the vehicle is not running or is running at a constant cruising speed, the drive shaft 52 is located at a neutral position in FIG. 11. In the neutral position, the torsional dampers are in the natural condition without swerving. A first maximum displaced position (PA) in FIG. 11 indicates the relative position of the drive shaft 52 with respect to the propeller shaft 51, when the torsional dampers are swerved (or twisted) within the maximum capacity thereof along the regular direction of the revolution of the drive shaft 51, in accordance with an engine acceleration. A second maximum displaced position (PB) indicates the relative position of the drive shaft 52 with respect to the propeller shaft 51, when the torsional dampers are swerved within the maximum capacity thereof along the reverse direction of the revolution of the drive shaft 51, in accordance with an engine deceleration.
When the engine is under the acceleration at timing t21 which is indicated in FIG. 12, the engine speed (NE) is thus increasing. The positive torque is applied on the torsional dampers, due to the acceleration. Consequently, the torsional dampers are swerved, and the drive shaft 52 is held at the first maximum displaced position (PA). In this case, the torsional dampers accumulate the repulsion force which causes the drive shaft 52 to return to the neutral position. This repulsion force acts to restrain the revolution of the drive shaft 52.
At timing t22 of FIG. 12, the engine speed (NE) exceeds the first determining value (Da) for the fuel injection halt. At timing t23 when the delay time .DELTA.t has elapsed since the timing t22, the controller transmits a signal to instruct the fuel cut-off operation. In the period of time (timing t22 through timing t23), the engine speed continuously increases.
When the fuel supply to the engine is cut off, the positive torque applied to the torsional dampers up to this point will be inverted to the negative torque. As a result, the drive shaft 52 shifts its position from the first maximum displaced position (PA) to the second maximum displaced position (PB). At the same time, the engine speed (NE) rapidly drops, that is caused by the repulsion force accumulated in the torsional dampers, in addition to the engine power drop originated in the fuel cut-off.
When the drive shaft 52 is at the second maximum displaced position (PB), the torsional dampers accumulate the repulsion force which acts on the drive shaft 52 to return to the neutral position. The repulsion force promotes the revolution of the drive shaft 52.
At timing t24 in FIG. 12, the engine speed (NE) drops below the second determining value (Db) for fuel injection resume. At timing t25 when the delay time .DELTA.t has elapsed since the timing t24, the controller transmits a signal to instruct to resume the fuel injection operation. During the period of time (between timings t24 and t25), the engine speed (NE) continuously decreases.
When the fuel injection operation is resumed, the negative torque applied on the torsional dampers up to this point is inverted to the positive torque. As a result, the drive shaft 52 shifts its position from the second maximum displaced position (PB) to the first maximum displaced position (PA). In this case, the engine speed (NE) rapidly increases, that is caused by the repulsion force accumulated in the torsional dampers, in addition to the rapid increase of the engine power which is generated by the resumption of fuel injection. In this manner, the controller repeatedly alternately carries out the halt and resume of the fuel injection operations.
The fluctuation of the engine speed (NE) per an unit time in the period of time after timing t23 is larger than that in the period of time before timing t23 (i.e., until the first fuel cut-off operation was carried out). Because, the engine speed is influenced by the repulsion force accumulated in the torsional dampers, after the first fuel cut-off operation was carried out. Therefore, even if the delay time .DELTA.t is kept constant, the amount of overshoot of the engine speed is gradually increased. As a result, the fluctuation of the engine speed will not be reduced, under the fuel cut-off control.